Kilz specifications only allow their products to be thinned by
10%--but they don't recommend thinning if trying to hide stains.
Make sure if your painter is using a Kilz product that he get's the
right one for your job. If it is an exterior job "KILZ Exterior" or
"KILZ Premium" is what is required. (Personally, I got better results
with BIN 123 for a latex exterior primer).
I never thinned paint with water, but occasionally I did thin paint
with "Floetrol". This is a product that slows down the drying time of
latex and lowers the amount of pressure needed to spray. In general,
Floetrol makes latex more like an alkyd to work with. It's a great
product and a lot better than diluting the paint with water, IMO.
It sounds like this painter is a school teacher on summer vacation
using his Wagner buzz gun to make some chump change.
Any professional with professional equipment will simply adjust the
tip size of the sprayer depending on the viscosity of the paint.
Most coatings specifically state "thinning not recommended". However
some will allow up to 10% thinning. Most coatings also recommend a
thickness spec. (too thin means they won't honor the warranty)
I'd be extremely concerned about any painter that wants to adjust the
product vs. adjust his equipment.
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